Book cage

A book cage refers to closed stacks that are locked off from public access by virtue of being placed inside a gated room (which usually looks like a jail cell). At the New York Public Library, this collection was often referred to as the "triple star" collection because all the call numbers started with "***." (Search CATNYP, the catalog of the research libraries of the New York Public Library, by call number using "***" to retrieve a list of these items.)